1. Field of the Invention
This invention relates to an image sensor and an electronic still camera which records an image signal outputted from the image sensor onto a recording medium such as a memory card.
2. Description of the Related Art
Electronic still cameras have such a general construction as shown in FIG. 7. Referring to FIG. 7, the electronic still camera shown includes a lens system 1, a shutter mechanism 2, and an image sensor 3 such as a CCD (charge-coupled device). A detected image signal outputted from the image sensor 3 is supplied to a processing circuit or camera signal processing circuit 4. The detected image signal is processed by and outputted from the processing circuit 4 in the form of primary color signals R, G and B of red, green and blue, respectively, or in the form of a brightness signal Y and a pair of color difference signals U and V.
The shutter mechanism 2 is opened or closed by a shutter drive circuit 6 under the control of a control circuit 5. An image is detected by the image sensor 3 in response to a shutter releasing operation, and thereupon a video signal is outputted from the processing circuit 4 and converted into a digital signal by an analog to digital (A/D) converter 7, whereafter it is supplied to a frame memory 8 which constitutes an image memory so that the video signal for one frame is written into the frame memory 8.
Then, the video signal for one frame read out from the frame memory 8 is compressed by an image compression circuit 9 and then written into a memory card 10 serving as a recording medium.
Though not described above, operation of the processing circuit 4, the frame memory 8, the image compression circuit 9, the memory card 10 and some other elements is controlled by the control circuit 5 while read pulses, transfer pulses and some other pulses required by the image sensor 3 and sampling pulses and some other necessary timings required by the analog to digital converter 7 are supplied from a timing generator 11.
An electronic still camera of the construction described above is disclosed in U.S. patent application Ser. No. 08/114,168 filed Sep. 1, 1993 and assigned to the same assignee.
In the image compression circuit 9, high efficiency compression based on, for example, the JPEG system is performed. Compression processing of the JPEG system involves orthogonal transformation processing such as DCT (discrete cosine transform). While such orthogonal transformation processing is normally performed by a digital circuit, such digital circuit is disadvantageous in that it is complicated because it involves a product sum calculator or a like element.